1. Field of the Invention
The present invention relates to a method and apparatus for monitoring the surface contact between a neutral electrode of a high-frequency (HF) surgical apparatus and a patient wherein the neutral electrode is split into at least two partial electrodes and wherein a comparison dependent alarm signal is generated.
2. Description of the Prior Art
HF-surgical apparatus is particularly useful for the stanching of blood during surgery. The oozing of blood is simply, quickly and safely stopped by the application of a spherical or flat electrode. Smaller vessels are grasped with a clamp and contacted with an active electrode; the vessel is sealed by coagulation and simultaneously bonded to the immediate area. The clamp may then be immediately removed. Thus, a ligature is no longer required in the case of small and medium sized vessels. (see Publication "Kleine Einfuehrung in die Elektrochirugie", published and available from Siemens AG, Erlangen, Federal Republic of Germany, Order No. ME 3990/1231, Pages 1 through 31).
The HF-current of a HF-surgical apparatus is normally a current flowing alternately in two directions. However, for the purpose of simplifying the following description, this HF-current will be treated as a direct current which flows from the HF-surgical apparatus, through a switch actuated by the operator to the active electrode, from there through the body of the patient to the neutral electrode, and from the neutral electrode back to the HF-signal apparatus.
In the ideal case the HF-current will flow at full intensity from the active electrode, through the patient and the neutral electrode and then back to the HF-surgical apparatus. In practice however, on occasion, so called "undesirable burns" result to the patient. The present invention deals with fault currents which flow through areas of limited or incomplete surface contact and lead to burns in these areas. These burns may occur at points where accessories are connected to the patient, but also at points where the neutral electrode does not make sufficiently close surface contact with the patient. The resulting burns may reach a degree where they become life threatening to the patient. The pamphlet "tkb" entitled "Sicherheitstechnische Anforderungen ("Technical Safety Requirements"), Pages 106 through 108), describes the various risks involved in the use of a HF-surgical apparatus having an isolated neutral electrode.
It is well known that patient safety is enhanced by dividing the neutral electrode into two partial electrodes. In such an arrangement, an auxiliary or measuring current is conducted from a measuring current source to one of the partial electrodes. From there, the current is conducted through the patient to the other one of the partial electrodes, and finally back to the measuring current source where it is monitored. When this current circuit is completed, it is established with certainty that essentially the total surface of each of the partial electrodes is in close contact with the patient and that the actual HF-working current may be applied. Although such a separated neutral electrode does indeed make it possible to determine whether proper contact is being made, it requires the application of a special auxiliary or measuring current.
As already mentioned, every effort must be made to avoid burns to the patient by the neutral electrode during HF-surgery. For the problem under consideration here, i.e., the risk of burns resulting from incomplete (reduced) surface area contact, an alarm signal is to be produced and/or safety means simultaneously actuated. Stated another way: Assurance against burns during the use of present day HF-surgical apparatus would be greatly enhanced if the surface area of the neutral electrode which is in close contact with the patient is adequately large. This will insure that the current density does not exceed a specified critical value. It is of importance, therefore, to monitor the totality of contact or the degree of surface area contact.
One method and circuit arrangement for measuring electrode contact area in electrosurgery devices is known from U.S. Pat. No. 4,200,104 (corresponding to DE-OS No. 2849422). In this known circuit arrangement, the neutral electrode is divided into two partial electrodes. The extent of the surface contact between the two partial electrodes and the patient is measured through an arrangement which directly determines the capacitance between the two partial electrodes. The capacitance measuring device includes a circuit for applying a signal between these two electrically conducting contact elements. The capacitance measuring device also includes means for determining whether the surface contact between the patient-double electrode and the patient exceeds a pre-determined value corresponding to the safe operation of the HF-surgical apparatus. It is known that the procedure for making the capacitance measurements involves difficulties and uncertainties and, moreover, requires a signal generator for providing the signal used for the capacitance measurement.
An object of the present invention is to provide a method and protective circuit arrangement which will make it possible to determine, with a high degree of safety, whether the neutral electrode of a HF-surgical apparatus contacts the patient with an adequately large surface area, without the necessity of making a capacitance measurement.